Is metformin safe for use during pregnancy?

Metformin Safety in Pregnancy

Metformin is not recommended as first-line therapy for gestational diabetes due to concerns about long-term offspring safety, though it may be considered as a second-line alternative to insulin in specific circumstances after thorough counseling about risks. 1

Primary Recommendation

Insulin remains the preferred medication for treating hyperglycemia in pregnancy. 1 The American Diabetes Association's 2024 Standards of Care explicitly states that metformin and glyburide should not be used as first-line agents because both cross the placenta to the fetus and data on long-term safety for offspring is of concern. 1

Key Safety Concerns

Placental Transfer

• Metformin readily crosses the placenta, resulting in umbilical cord blood levels as high or higher than simultaneous maternal levels. 1
• This complete placental transfer exposes the developing fetus to therapeutic maternal doses throughout pregnancy. 1

Long-Term Offspring Effects

The most significant concern involves childhood metabolic outcomes:

• In the MiG TOFU study, 9-year-old offspring exposed to metformin for GDM treatment in the Auckland cohort were heavier and had higher waist-to-height ratios and waist circumferences than insulin-exposed children. 1
• In PCOS studies, 4-year-old offspring showed higher BMI and increased obesity, with effects persisting at 5-10 years (higher BMI, weight-to-height ratios, waist circumferences, and borderline increased fat mass). 1
• Meta-analyses demonstrate metformin exposure results in smaller neonates with acceleration of postnatal growth, resulting in higher BMI in childhood. 1
• Notably, the MiTy Kids trial showed no differences in anthropometrics at 24 months, suggesting variability in outcomes. 1

Short-Term Maternal Benefits

Despite long-term concerns, metformin does offer some maternal advantages:

• Lower risk of neonatal hypoglycemia compared to insulin. 1
• Less maternal weight gain during pregnancy. 1
• However, treatment failure occurs in 14-46% of individuals requiring insulin supplementation. 1

When Metformin May Be Considered

Acceptable Second-Line Scenarios

Metformin may be an alternative for individuals with GDM who cannot use insulin safely or effectively due to:

• Cost barriers
• Language barriers
• Comprehension difficulties
• Cultural influences

This requires thorough discussion of known risks and need for more long-term safety data in offspring. 1

Absolute Contraindications in Pregnancy

Metformin should NOT be used in pregnant individuals with: 1

• Hypertension
• Preeclampsia
• Risk for intrauterine growth restriction
• Placental insufficiency (due to potential for growth restriction or acidosis)

Specific Clinical Contexts

Polycystic Ovary Syndrome (PCOS)

Metformin used for ovulation induction should be discontinued by the end of the first trimester. 1 Randomized controlled trials show no evidence-based benefit in preventing spontaneous abortion or GDM when continued beyond this point. 1

Type 2 Diabetes in Pregnancy

RCTs comparing insulin alone versus insulin plus metformin showed no differences in composite neonatal health outcomes, though neonatal birth weights were smaller in metformin groups with increased small-for-gestational-age neonates. 1

Prevention of GDM

Metformin does not reduce the risk of GDM in high-risk individuals with obesity, PCOS, or preexisting insulin resistance. 1

FDA Labeling Perspective

The FDA label states that limited data with metformin in pregnant women are not sufficient to determine drug-associated risk for major birth defects or miscarriage. 2 Published post-marketing studies have not reported clear associations with major birth defects, miscarriage, or adverse maternal/fetal outcomes, though these studies cannot definitively establish absence of risk due to methodological limitations. 2

Clinical Decision Algorithm

1. First-line treatment: Lifestyle modification (diet, exercise) for all pregnant individuals with GDM 1
2. If pharmacotherapy needed: Insulin is preferred 1
3. Screen for metformin contraindications: Hypertension, preeclampsia, risk of intrauterine growth restriction 1
4. If insulin not feasible AND no contraindications exist: Consider metformin only after detailed counseling about:
   • Placental transfer to fetus 1
   • Potential childhood metabolic effects (increased BMI, adiposity) 1
   • 14-46% treatment failure rate requiring insulin addition 1
   • Need for long-term offspring monitoring 3, 4

Important Caveats

• Recent evidence suggests metformin does not increase risk of preterm labor or small-for-gestational-age infants compared to diet control alone in some populations. 5
• A 2024 target trial emulation found little to no increased risk for nonlive birth when continuing metformin with insulin versus switching to insulin monotherapy. 6
• The evidence base remains limited by heterogeneous study designs, high attrition rates, and inadequate control for confounding variables. 3